Adjustable valve pin assembly

ABSTRACT

The valve pin adjustment system is provided for use in valve gated injection molding applications which enables adjustments to the positioning of the valve pin without necessitating removal of the clamping plates and valve assembly components, including valve pins. A piston associated with the valve pin includes a cylindrical body having first and second co-axially aligned internally threaded bores of respective first and second diameters. Preferably, the threads formed in the first and second bores are of the opposite hand. First and second externally-threaded cylindrical valve pin positioning members are respectively received in the first and second bores. Each of the positioning members includes a head portion and a tip portion. The tip of the second positioning member is in contact with the head of the first positioning member to lock the first positioning member in place, and the tip of the first positioning member is in contact with a head portion on the valve pin to locate the valve pin.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of provisionalapplication Ser. No. 60/480,777, filed on Jun. 23, 2003.

BACKGROUND OF THE INVENTION

The present invention generally relates to valve gate systems wherein areciprocating valve pin opens and closes a melt passageway in injectionmolding applications. More particularly, this invention relates to valvepin assembly and a method for adjusting valve pin position to ensureproper opening and closing of the melt passageway during injectionmolding operations.

Valve gated injection molding systems are known in which pistons areactuated pneumatically or hydraulically to reciprocate the valve member.In general, such systems require that the components, in particular thevalve pin and its associated piston, be made to very close tolerances toallow for thermal expansion so that, when at operating temperatures, thefront end of the valve pin seats precisely in the gate to close it. Ifthe valve pin does not extend far enough into the gate, it will not seattightly when in the closed position, leading to a poor gate vestige. Ifthe valve pin extends too far toward the gate, the tip of the pin willrepeatedly impact the gate on closing, thus causing undue wear of boththe valve pin and the gate and, potentially, fracture of the mold aroundthe gate.

Typically, final adjustment of the positioning of the valve pin is acomplicated and time-consuming procedure. First, the hot runner systemin which the valve pins are to be used is fitted to the mold plate(s)without the upper clamp plate and valve pins in place. The valve pins,which have been cut to a calculated length, are inserted into theassembly. A depth gauge is used to measure the position of the valve pinhead at room temperature from a reference point, usually the backsurface of the mold plate. The assembly is then heated to the processingtemperature and held at that temperature for a period of time sufficientfor all the hot runner components, including the valve pins, to beuniformly heated. The depth gauge is then used to measure the positionof the valve pin head at operating temperature (the valve pin head's newposition), and the amount of the adjustment of each valve pin iscalculated based upon the differences of the two measurements.Specifically, if the valve pin is too long, it must be removed andshortened; if the valve pin is too short, the piston that carries thevalve pin is removed and its bottom is ground to shorten the piston.This measurement and shortening/lengthening process is repeated for eachvalve pin in the system. It is, obviously, an exacting and timeconsuming process.

Accordingly, it is an object of the present invention to provide animproved valve pin assembly and method for adjusting the valve pin thatfacilitates the quick and precise positioning of the tip of the valvepin with respect to the mold gate.

SUMMARY OF THE INVENTION

In accordance with this invention, a novel valve pin adjustment systemis provided for use in valve gate injection molding applications whichenables adjustments to the positioning of the valve pin withoutnecessitating removal of manifold plates and valve assembly components,including the valve pins. A piston that captures the head of the valvepin includes a cylindrical body having upper and lower internallythreaded bores of respective first and second diameters. Preferably theupper bore is oppositely threaded with respect to the lower bore, andthe upper and lower bores are in axial alignment with each other andwith the valve pin. First and second externally threaded cylindricalvalve pin positioning members are respectively threadedly received inthe lower and upper bores. Each of the threaded positioning membersincludes a head portion and a tip portion. The head portions are eachprovided with a hex-shaped recess or equivalent tool-engaging surfacefor receiving a hex key wrench or the like. The tip of the firstpositioning member is in contact with the head portion of the valve pinto locate the valve pin, while the tip of the second positioning memberis in contact with the head of the first positioning member to lock thefirst positioning member in place, so that first positioning member isrestrained from unscrewing.

In use, and in contrast to the prior art, the entire hot half of the hotrunner system is assembled, with the hot runner manifold completelyenclosed and the valve pins in place with their tips in contact with thegates. As such, the system is fully functional, with all the valve gateassemblies operable. The hot runner system is then heated to operatingtemperature, thus causing all the components to expand to theiroperating size so that this expansion can be taken into account in thefinal positioning of the valve pins. The valve pins are gently tapped ontheir heads to insure that the tips are in contact with their respectivegates, and, for each valve pin, the first positioning member is screwedinto its piston until its tip contacts the head of the valve pin topositively locate the valve pin in its closed condition. Then the secondpositioning member is screwed into the piston in the opposite directionfrom the first positioning member until the tip of the secondpositioning member engages the head of the first positioning member,thus locking the first positioning member in place. A specially-designedwrench may be used to facilitate installing the positioning members. Thecover plates for each valve pin assembly are then attached to the topclamp plate to complete the process.

In another aspect of the invention, the piston is received in a cylinderthat also serves as the pressure pad and insulator between the top clampplate and the manifold of the hot runner system. The well in the topclamp plate which overlies the cylinder and piston includes one or moreradially-extending slots that, upon rotation of the piston, align withsimilarly radially-extending slots on the upper end of the piston. Thespecially-designed wrench may then be used which has one or more bladesthat engage the aligned slots, thus preventing rotation of the pistonrelative to the top clamp plate when the first and second threadedpositioning members are being screwed into the piston.

In still another aspect of the invention, the bores in the piston andthe cylinder which receives the shaft of the valve pin are oversizedwith respect to the diameter of the shaft of the valve pin toaccommodate the lateral expansion of the manifold with respect to themold center.

In a still further aspect of the invention, the shaft of the valve pinhas one or more circumferential grooves that serve as a repository forany melt that may migrate through the manifold toward the cylinder andtop plate.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described in conjunction with theaccompanying drawings in which like reference numerals designate likeparts and wherein:

FIG. 1 is a perspective view of the assembled clamping plates and hotrunner manifold (not seen) for use with the valve pin adjustment systemof the present invention;

FIG. 2 is a side elevational view, partly in section, of assembledclamping plates, hot runner manifold and nozzle showing an embodiment ofthe valve pin adjustment system of the present invention;

FIG. 3 is fragmentary plan view of the top clamp plate with the accessplate removed to show the well for receipt of the valve pin;

FIG. 3A is a perspective view of a specially designed tool for use inadjusting the position of the valve pin of the present invention.

FIG. 4 is an enlarged sectional view of the valve pin adjustment systemshown in FIG. 2;

FIG. 5 is a perspective view of the piston of the valve pin adjustmentsystem of the present invention showing the lower portion thereof;

FIG. 6 is a perspective view of the piston shown in FIG. 5 showing theinternal threads in the upper bore thereof;

FIG. 7 is a perspective view of the cylinder of the valve pin adjustmentsystem of the present invention wherein a portion of the upper interiorportion thereof is viewable;

FIG. 8 is a perspective view of the cylinder shown in FIG. 7 in aninverted position.

FIG. 9 is a perspective view of the upper valve pin positioning member;and

FIG. 10 is a perspective view of the lower valve pin positioning member.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, and, in particular with reference to FIG. 1,a manifold system for a multi-cavity injection molding apparatus isgenerally depicted by the reference numeral 10 having a manifold plate11 and a clamp plate 12. The hot runner manifold (not seen in FIG. 1) isclamped between the two plates 11 and 12, which are secured together bya plurality of bolts. The clamp plate 12 includes a locating ring 13athat fits into the bore of a platen of the molding machine fordelivering pressurized melt to the system through an intake nozzle 13 onthe manifold. A plurality of nozzle-access compartments or wells 14a,14b, 14c and 14d (one for each nozzle) is provided. The associated coverplates are shown removed and are respectively designated by thereference numerals 15a, 15b, 15c and 15d. While the illustratedembodiment shows a system with four nozzles, any number of nozzles maybe used, depending upon the particular application.

As is generally shown in FIG. 2, a heated nozzle 16 is provided having anozzle seal 17 at the front end thereof, an internal melt-flow bore 18,and an elongated valve pin 19. The valve pin 19 comprises a shaftterminating in a tip 19a positioned to be selectively moved into openand closed positions with respect to a gate 21. The gate 21 communicateswith a mold cavity 50 in a mold (shown in phantom) for alternatelyadmitting melt into the cavity 50 and shutting-off the flow of melt.While the tip 19a is shown to be shaped with a taper and lapped, it isunderstood that it may be of various other shapes, such as circular,without departing from the invention.

Elongated pins 32 are provided to both locate and align each plate 11and 12 with respect to each other and to align the assembled manifoldsystem 10 with respect to its associated mold. The positioning pins 32are installed in their respective holes in the manifold plate 11 priorto the placement of the clamp plate 12 thereon. The positioning pins 32are sufficiently long to insure that the bore in the clamp plate 12 thatreceives the actuating piston and cylinder for the valve pin (describedbelow) is properly aligned before the surfaces of the clamp plate 12defining the bore come into contact with either the piston or cylinder.Thus, potential damage to the piston and cylinder during assembly due tothe misalignment is avoided.

Similarly, the length of the locating pins 32 that extends beyond themanifold plate 11 is preferably greater than the length of the portionof the nozzle(s) 16 that extends out of the manifold plate 11. Thisinsures that each nozzle seal 17 is properly aligned with its intendedwell in the mold before the system is assembled to the mold, thusavoiding potential damage to the nozzle seal(s) 17 due to misalignment.

Referring to FIGS. 2 and 4, a hot runner manifold 33 is shown which isclamped into position between the two plates 11 and 12 by means of apressure pad/insulator 23 and the head of nozzle 16. Specifically, thepressure pad/insulator 23 engages the top side of the manifold 33, whilethe back end of the nozzle 16 engages the bottom side of the manifold33. As set forth below, the pressure pad/insulator 23 also serves as thecylinder for the valve pin piston.

In keeping with one aspect of the invention, a piston 22 is providedwhich is received for reciprocal movement within the cylinder 23.Advantageously, the pressure that is exerted on the top of the cylinder23 by the top clamp plate 12 due to the expansion of the manifold 33 issuch that an air-tight seal is created between the two. The lower end ofthe cylinder 23 forms a seal with respect to the top clamp 12 by meansof an elastomeric O-ring 23b received in annular groove 23a in the outerwall of the lower portion of cylinder 23, thus allowing for apressurized air to be admitted to the lower portion of the cylinder.Importantly, at least a portion of the cylinder wall forms acircumferential space 51 with respect to the clamp plate 12 tofacilitate delivery of pressurized air to the interior of the cylinder.To this end, the lower chamber 26 of the cylinder 23 communicates withthe space 51 by means of a plurality of air supply inlets 27 which arecircumferentially disposed around the lower end of the cylinder 23 (bestseen in FIG. 7). Similarly, the upper chamber 25 is supplied by a secondair supply inlet 7 (shown in phantom in FIG. 4). Additionally, thepiston 22 is provided with an annular ring seal 24 to form a pneumaticseal. Thus, a differential air-pressure can accumulate across the piston22 between an upper chamber 25 and a lower chamber 26 in cylinder 23. Bysupplying pressurized air in alternating fashion to the chambers 25 and26, reciprocating movement of the piston 22 is selectively provided.

In keeping with another aspect of the invention, threaded members areprovided which are received in the piston 22 for properly positioningand locking the valve pin 19 into position, once properly located. Tothis end, an upper cylindrical portion 22a of piston 22 includes anupper bore 22b and a lower bore 22c. As illustrated, the upper and lowerbores 22b, 22c are in axial alignment with each other. However, axialalignment is not required so long as access to the lower bore can beobtained through the upper bore for insertion of a first positioningmember (described below). In the illustrated embodiment, upper bore 22bis shown to have a larger diameter than the lower bore 22c. Each ofupper and lower bores 22b and 22c is provided with its own internalthreads. Preferably, the threads on upper bore 22b are in an oppositedirection to the lower bore 22c (e.g. upper bore 22b is provided with aleft-hand thread and lower bore 22c is provided with a right-handthread).

A first externally threaded cylindrical valve pin positioning member 29,which is in axial alignment with the valve pin 19 is received within thelower bore 22c. As shown, the first positioning member 29 includes arecess 29a, preferably hex-shaped, in a head portion thereof which issized to receive a suitable hex wrench for screwing the firstpositioning member 29 into the lower threaded bore 22c. As shown, a tipportion 29b includes a flat end-face adapted for contact with theend-face of the valve pin head 19b.

A second externally threaded cylindrical valve pin positioning member 28having a head portion 28a and a tip portion 28b is received within upperbore 22b. In the illustrated embodiment, head portion 28a includes asocket, also preferably hex-shaped, for receiving a suitable hex wrenchor equivalent tool for screwing the second positioning member 28 intothe threaded bore 22b. When screwed into the upper bore 22b, the secondpositioning member 28 serves to fix the location of the firstpositioning member 29 in the piston 22. Because the first and secondpositioning members are received in bores having opposite internalthreads, they will not loosen or unscrew over time, thus maintaining theproper positioning of the valve pin.

The area immediately above the second valve pin positioning member 28includes a chamber 30 which communicates with the atmosphere via hole52. In accordance with an important aspect of the present invention, thecylindrical portion 22a of piston 22 is sealingly retained in the clampplate 12 by means of an elastomeric seal 31 which is fitted between theclamp plate 12 and outer wall of the upper cylindrical portion 22a asshown.

Thus, in order to install the valve pin in an injection moldingapparatus having a piston and positioning members as described above,the cover plate on the top clamp plate is removed to expose the well andprovide access to the cylinder. The valve pin, with its tip alreadyhaving been shaped and lapped, is then inserted through the piston andcylinder so that the valve pin extends through the melt passage with itstip in contact with the gate.

The system is then heated to its operating temperature, thus causing thevalve pin to expand to its operating length, as well as causing theother components of the systems to expand to their operating size. Ascan be appreciated, the valve pin assembly, once raised to operatingtemperature, is functional at this point, as the top clamp plate is inposition secured to the lower clamp plate, and pressurized air can bedelivered to the cylinder. The valve pin head is then gently tapped toinsure that the tip is in sealing contact with the gate. If the tip ofthe valve pin fails to contact the gate due to the valve pin being tooshort, the bottom surface of the piston can be machined to effectivelylengthen the shaft of the valve pin. This would require disassemblingthe clamp plate from the system, which is highly undesirable and resultsin the disadvantages associated with the prior art. Alternatively, thelower face of the valve pin can be machined to effectively lengthen theshaft. Significantly, the present invention permits the use of valvepins with overly long shafts without requiring machining of the valvepin to properly position the tip. Specifically, if the valve pin is toolong, a space results between the lower surface of the head of the valvepin and the opposed surface on the piston when the valve pin closes thegate (as best seen in FIG. 4). Thus, when the valve pin is moved fromits retracted position to the closed position to the gate, the valve pinfloats until its head is contacted by the first positioning member.Accordingly, it is preferred to intentionally make the valve pinsoversized in length. Then the piston will not need to be removed or thelower face of the valve pin machined in order to properly position thevalve pin.

The first externally-threaded positioning member is then screwed intothe lower internally-threaded bore in the piston until it contacts thehead of the valve pin, but not farther, so that it serves as a positivestop to prevent the valve pin from backing up short of the gate. Thisinsures that, during operation, the valve pin will not extend too fartoward the gate, yet will firmly close the gate to achieve a good gatevestige. Then the second externally-threaded positioning member isscrewed in the opposite direction into the upper internally-threadedbore in the piston until the tip portion engages the head portion of thefirst positioning member interior of the socket in the head. This locksthe first positioning member in place. The cover plate can then bereattached to the top plate. This general procedure is repeated for eachnozzle in the system.

In keeping with another aspect of the invention, means are provided forpreventing rotation of the piston 22 with respect to the well 14a whenthe first and second positioning members 28 and 29 are screwed into thepiston. With reference to FIGS. 5 and 6, the top end of the piston 22includes one or more radially-extending notches 34 (four shown) that aresubstantially equally spaced about the upper edge of the piston 22. Asbest seen in FIG. 3, the well 14a includes a corresponding number ofradially-extending slots 35 having a depth and width similar to thenotches 34 in the top of the piston 22. In order to prevent rotation ofthe piston 22 with respect to the well 14a during the placement of thefirst or second positioning members, and to apply torque to thepositioning members, a specially-designed tool 41, shown in FIG. 3A, maybe used which has a blade-like portion 42 that is received in both thenotches 34 and slots 35 aligned therewith, thus preventing relativerotation. As can be appreciated, the tool 41 has a central bore 43through which a further tool, i.e., a hex wrench, can be inserted thatwill engage the sockets in the head portions of the positioning members.Alternatively, the slots 35 in the well 14a may be eliminated, while thenotches 34 in the top of the piston 22 are retained. The blades 41 ofthe tool 41 are then located so as to engage only the notches 34 in thepiston 22. In this case, the tool 40 is gripped by the installer toprevent rotation of the piston 22, in contrast to the illustratedembodiment in which the tool 40 does not need to be held duringinstallation of the positioning members.

In keeping with another aspect of the invention, both the piston 22 andthe cylinder 23 have an oversized bore, 36 and 37, respectively, forreceiving the shaft of the valve pin 19 (best seen in FIG. 4). Incontrast, the diameter of the bore in the manifold 33 for receiving theshaft valve pin 19 provides for a very close fit. The enlarged bores 36,37 allow some lateral shifting of the valve pin 19 upon lateralexpansion of the manifold 33 with respect to the mold center during thecycling of the system, thus reducing the likelihood of the valve pinshafts being bent or flexed.

In a further aspect of the invention, the valve pin 19 is provided withone or more circumferential grooves 38 (best seen in FIG. 4, threeshown), which are located such that during reciprocation of the valvepin, they move between a position in which the grooves 38 are locatedwithin the manifold seal 39 and a chamber 40 formed between the lowerend of the cylinder 23 and the manifold 33 (best seen in FIG. 4).Preferably, the upper surface of the manifold 33 carries a manifold seal39, through which the valve pin 19 passes. The circumferential grooves38 in the valve pin 19 serve as a repository for any melt that maymigrate from the melt flow bore 18 up the shaft on the valve pin pastthe seal 39. As the valve pin 19 is reciprocated, and thecircumferential rings 38 move between the chamber 40 and the seal 39,any melt that has collected in the circumferential grooves 38 will besheared off and collect in the chamber 40.

Thus, an improved adjustable valve pin assembly has been provided thatmeets the object of the present invention. While the invention has beendescribed in terms of a preferred embodiment, there is no intent tolimit it to the same. Instead, the scope of the invention is defined bythe following claims.

1. A valve gated injection molding apparatus comprising: a nozzle havinga melt passage therethrough extending to a gate; a valve pin having anelongated shaft with a front end terminating in a tip and a rear endterminating in an enlarged head; a piston adapted to capture the head ofthe valve pin, the elongated shaft of the valve pin extending throughthe piston into the melt passage to the gate, the piston comprising abody having first and second internally-threaded bores of respectivefirst and second diameters; and first and second externally-threadedvalve pin positioning members adapted to be received in the first andsecond bores in the body of the piston, the first positioning memberengaging the head of the valve pin and the second positioning membercontacting the first positioning member to lock the first positioningmember in place.
 2. The valve gated injection molding apparatus of claim1 wherein the first and second bores in the body of the piston areoppositely threaded.
 3. The valve gated injection molding apparatus ofclaim 1 wherein each of the first and second positioning members has ahead and a tip, the head being configured with a surface adapted to beengaged by a torque-applying tool, the tip of the first positioningmember adapted to contact the valve pin, and the tip of the secondpositioning member being adapted to engage the head of the firstpositioning member.
 4. The valve gated injection molding apparatus ofclaim 3 wherein the heads of the first and second positioning membersare recessed and the tip of the second positioning member is sized to bereceived in the recess in the head of the first positioning member. 5.The valve gated injection molding apparatus of claim 1 wherein thepiston has at least one slot formed therein adapted to be engaged by atool so as to prevent rotation of the piston.
 6. A valve gated injectionmolding apparatus comprising: at least one nozzle having a meltpassageway therethrough extending to a gate; a manifold to deliver meltto the nozzle; first and second clamp plates to enclose the manifold andmaintain the nozzle in contact therewith; a cylinder positioned betweenthe first clamp plate and the manifold in alignment with the nozzle; avalve pin having an elongated shaft with a front end terminating in atip and a rear end terminating in an enlarged head; a piston adapted tobe received in the cylinder for capturing the head of the valve pin, theelongated shaft of the valve pin extending through the piston into themelt passage to the gate, the piston comprising a cylindrical bodyhaving first and second internally-threaded bores of respected first andsecond diameters; first and second externally-threaded valve pinpositioning members adapted to be received in the first and second boresin the body of the piston, the first positioning member engaging thehead of the valve pin and the second positioning member contacting thefirst positioning member to lock the first positioning member in place.7. The valve gated injection molding apparatus of claim 6 wherein thefirst clamp plate includes a well through which access to theinternally-threaded bores in the body of the piston is obtained, each ofthe well and the body of the piston having at least one slot, the slotsbeing alignable upon rotation of the piston so as to receive a tool toprevent rotation of the piston with respect to the first clamp plate andto apply torque to the positioning members.
 8. A valve gated injectionmolding apparatus comprising: a nozzle having a melt passagetherethrough extending to a gate; a valve pin having an elongated shaftwith a front end terminating in a tip and a rear end terminating in anenlarged head; a piston adapted to capture the head of the valve pin,the elongated shaft of the valve pin extending through the piston intothe melt passage to the gate, the piston comprising a body having atleast one internally-threaded bore; and first and secondexternally-threaded valve pin positioning members, at least one of saidvalve pin positioning members being threadedly received in the at leastone internally-threaded bore in the body of the piston, the firstpositioning member engaging the head of the valve pin and the secondpositioning member contacting the first positioning member to lock thefirst positioning member in place.
 9. The valve gated injection moldingapparatus of claim 8 wherein each of the first and second positioningmembers has a head and a tip, each said head being configured with asurface adapted to be engaged by a torque-applying tool, the tip of thefirst positioning member adapted to contact the head of said valve pin,and the tip of the second positioning member being adapted to engage thehead of the first positioning member.
 10. The valve gated injectionmolding apparatus of claim 9 wherein the head of the first positioningmember is recessed and the tip of the second positioning member is sizedto be received in the recess in the head of the first positioningmember.
 11. The valve gated injection molding apparatus of claim 1wherein the piston has at least one slot formed therein adapted to beengaged by a tool so as to prevent rotation of the piston.
 12. A valvegated injection molding apparatus comprising: at least one nozzle havinga melt passageway therethrough extending to a gate; a manifold todeliver melt to the nozzle; first and second clamp plates to enclose themanifold and maintain the nozzle in contact therewith; a cylinderpositioned between the first clamp plate and the manifold in alignmentwith the nozzle; a valve pin having an elongated shaft with a front endterminating in a tip and a rear end terminating in an enlarged head; apiston received in the cylinder for capturing the head of the valve pin,the elongated shaft of the valve pin extending through the piston andmanifold and into the melt passage to the gate, the piston comprising acylindrical body having at least one internally threaded bore; first andsecond externally-threaded valve pin positioning members, at least oneof said valve positioning members being threadedly received in said atleast one internally-threaded bore in the body of the piston, the firstpositioning member engaging the head of the valve pin and the secondpositioning member contacting the first positioning member to lock thefirst positioning member in place.
 13. The valve gated injection moldingapparatus of claim 12 wherein the first clamp plate includes a wellthrough which access to the at least one internally-threaded bore in thebody of the piston is obtained, each of the well and the body of thepiston having at least one slot, said slots being alignable uponrotation of the piston so as to receive a tool to prevent rotation ofthe piston with respect to the first clamp plate the application oftorque to the positioning members.
 14. The valve gated injection moldingapparatus of claim 12 wherein the first clamp plate includes a wellthrough which access to the at least one internally-threaded bore in thebody of the piston is obtained.
 15. The valve gated injection moldingapparatus of claim 12 wherein the body of the piston has a upper endwhich includes a radial slot that is adapted to receive a tool toprevent rotation of the piston during the application of torque to thepositioning members.
 16. A valve gated injection molding apparatuscomprising: a nozzle having a melt passage therethrough extending to agate; a valve pin having an elongated shaft with a front end terminationin a tip which is adapted to selectively shut off the flow of melt intothe gate and a rear end terminating in an enlarged head; a cylinder inan overlying relation to said nozzle; a piston in said cylinder mountedfor reciprocal movement therein, said piston including a cylindricalwall defining a generally cup-shaped chamber which includes at least oneinternally threaded bore and a bottom end wall through which the valvepin shaft extends; first and second axially aligned externally threadedvalve pin positioning members contained within said chamber togetherwith the enlarged head of said valve pin which is captured in the spacebetween an end face of said first valve pin positioning member and thebottom end wall of said chamber, said valve pin head being sized so thatit has an axial length between a top face and a bottom collar thereofwhich is less than the axial distance between said end face of saidfirst valve pin positioning member and the bottom end wall of saidchamber, whereby when said piston is actuated to move said valve pin toshut off the flow of melt into the gate, the end face of said firstvalve pin positioning member will be in contact with the top end face ofsaid valve pin head and an open space will be present between the bottomcollar of said valve pin head and the end wall of said chamber and,correspondingly, when said piston is actuated to move said valve pin topermit the flow of melt into said gate, said bottom collar of said valvepin head will be in contact with the end wall of said chamber to providean open space between the top end face of said valve pin head and theend face of said first valve pin positioning member.
 17. The valve gatedinjection molding apparatus of claim 16 wherein said generallycup-shaped cylindrical chamber includes a second threaded bore inoverlying axially aligned relation to said at least one internallythreaded bore.
 18. The valve gated injection molding apparatus of claim17 wherein said second threaded bore has a larger diameter than said atleast one internally threaded bore.
 19. The valve gated injectionmolding apparatus of claim 18 wherein said second threaded bore isoppositely threaded with respect to the threads of said at least oneinternally threaded bore.
 20. The valve gated injection moldingapparatus of claim 16 wherein first and second clamp plates enclose saidmanifold and maintain the nozzle in contact therewith and said firstclamp plate includes a well through which access to said generallycup-shaped chamber is obtained.